1. Field of the Invention
The present invention relates to a MAG (metal active gas) welding method of a galvanized steel sheet wherein corrosion proofing is provided on the surface of the steel sheet.
Priority is claimed on Japanese Patent Application No. 2005-091198, filed Mar. 28, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
In the fields of components and equipment pieces of associated with and/or used for automobiles, business equipment and the like, a galvanized steel sheet is widely used due to its excellent corrosion resistance and luster thereof. The galvanized steel sheet is a sheet in which galvanization is conducted on an iron base surface of a steel sheet.
However, there is a problem in that many pits and blowholes are generated when MAG welding is conducted on a galvanized steel sheet(s). The reason is that, since zinc has a melting point (419° C.) and boiling point (907° C.), which are lower than those of steel having a melting point (1535° C.) and boiling point (2750° C.), when zinc enters in a molten pool while welding is conducted, zinc vapor generated from the zinc is caught in the molten pool together with air, and blowholes are formed when said air and zinc vapor fail to reach the surface before the solidifying process of the welded metal is completed. When such pits are produced, a problem arises in that strength which should be achieved by the welding inherently cannot be obtained. Therefore, when pits are generated, repair of the portion where pits are generated is conducted. Furthermore, when a galvanized steel sheet is welded at high speed, further large numbers of pits, blowholes and the like are formed, and this is unpreferable.
In order to solve the problems, the following methods and the like have been conducted in general.
(1) Zinc is removed mechanically in advance from a line on which welding should be conducted. However, in the method, the number of steps for welding increase due to the addition of steps for removing zinc from the line, and this causes a problem in that the cost increases due to the addition of the steps.
(2) Occurrence of pits is prevented such that a zinc vapor generated is prevented from moving to the surface of the metal by using, for example, a welding wire, which comprises increased amounts of Si, Mn and the like, for welding a galvanized steel sheet in order to increase the viscosity of the molten metal. On the other hand, occurrence of pits and blowholes is prevented such that, for example, a welding wire comprising decreased amounts of Si, Mn and the like is used for welding a galvanized steel sheet in order to decrease the viscosity of the molten metal so that a zinc vapor rise to the surface of the metal easily. However, in fact, such wires do not provide excellent effects for preventing the generation of pits and/or blowholes.
(3) A method is proposed, for example, in Japanese Patent No. 2668125, wherein generation of pits and blowholes is prevented by using a mixed gas consisting of oxygen, carbon dioxide, and argon, which occupies the residual volume of the mixed gas, for welding. The mixed gas is used in order to prevent an occurrence of a zinc vapor by the effect of oxidation of zinc caused by oxygen. Furthermore, a method is proposed wherein generation of pits and blowholes is prevented due to the effect that surface tension of the molten metal is decreased due to oxygen and gases pores included in a molten metal rise to the surface easily. However, the disclosed welding speed thereof is only 120 cm/min.
Accordingly, the purpose of the present invention is to provide a shielding gas which does not generate pits and blowholes in a welded portion when a lap fillet welding or the like of a galvanized steel sheet is conducted in a MAG welding method.